1. Field of the Invention
The present invention relates to amplifiers and, in particular, to microphone preamplifiers used to preamplify signals received from microphones.
2. Description of the Related Art
The use of amplifiers is ubiquitous today. One type of amplifier, known as a "preamplifier," is often used to amplify ("preamplify") a raw, weak signal received from some source, such as a transducer. In one usage, the comparatively small voltage input signal is provided by a microphone to an amplifier referred to as a microphone preamplifier. The microphone preamplifier amplifies the input analog audio signal to provide a larger-amplitude "preamplified" output signal having the same waveform as that of the input signal, within a certain degree of accuracy. The preamplifier typically provides a preamplified signal which is within a standardized dynamic range.
The preamplified signal is then passed on to some further stage. For example, the preamplified signal may be processed by fuer components, such as automatic gain control (AGC) or other components or stages. The preamplified signal may be further amplified by an amplifier, to drive an output device such as a loudspeaker, for example. Other uses may also be made of the preamplified signal. For example, the preamplified signal may be further processed or amplified and digitized and/or recorded on some storage medium in digital or analog form. If converted to digital form, the digital audio signal may be processed by a digital signal processor (DSP) before being stored or played on a speaker.
Because the voltage signal input into preamplifiers is weak, it is important for microphone preamplifiers, as well as other amplifiers amplifying weak input signals, to have a very good noise performance. Otherwise, the noise, which may be significant in comparison to the small input signal, may dominate. Noise arises in a variety of ways. For example, in amplifiers (including preamplifiers) implemented as part of an integrated circuit (IC) having a substrate, noise can arise from comparatively noisy sections of the substrate and thus adversely affect the amplifier portions of the IC. Pass-transistor switches turning off in switched-capacitor applications may also produce noise. Noise may be generated when unavoidable parasitics, associated with all silicon ICs, provide numerous paths for unwanted disturbances to couple into the signal path of an analog circuit via the substrate, the power supply rails, the ground lines, and/or even directly from nonideal components. Noise may accompany the input signal if the input lines delivering the input signal from the transducer pick up noise from some source external to the IC. Such noise may come, for example, from a noisy power supply that powers a transducer such as a microphone. Feedback resistors may also introduce thermal noise into the input signal path. the input stage of the amplifier itself may also introduce thermal noise and so-called "flick" noise.
Noise may thus be introduced into the signal path of an amplifier (or preamplifier), and be amplified along with the input signal, thus causing the amplified (preamplified) output signal to be a distorted representation of the input waveform. Such disturbances and distortions can accumulate, potentially leading to serious loss in signal-to-noise ratio and dynamic range.
Various types of amplifiers are in use. ICs typically implement amplifiers which utilize one or more operational amplifiers ("op amps"). A conventional single-ended op amp, which has differential input and singled-ended output, may be especially prone to being adversely affected by such noise. Such an op amp has positive and negative differential inputs, and a single output terminal that provides an output voltage with respect to ground.
More complicated amplifier configurations, such as differential output op amp circuits or balanced differential output op amp circuits, are often utilized because of their superior noise resistance characteristics. For example, a differential op amp maintains positive and negative signal paths and provides two differential output terminals rather than having a single-ended output. The output voltage is the difference between the two differential output terminals rather than an absolute value with respect to ground. This can help reduce the impact of noise, such as that produced by parasitic couplings or other sources. For example, if such noise is injected into one signal path it is likely that the same or similar noise will be injected into the other signal path. Thus, since the output signal is seen as the difference between the two output terminals, the effect of the noise will be canceled out and will not be present in the output signal itself.
Further improvement is possible if such an analog op amp circuit is not only differential, but also balanced. A balanced differential op amp circuit is realized with dual inverting and noninverting signal paths, in a completely symmetrical layout, such that all parasitic injections couple equally into both signal paths as common-mode signals. The differential nature of these circuits causes these common-mode disturbances to cancel (or nearly cancel) such that their impact is reduced significantly. Single-ended, differential output, and balanced differential output op amp circuits are described in further detail in David A. Johns & Ken Martin, Analog Integrated Circuit Design (New York: John Wiley & Sons, Inc., 1997): pp. 280-282, and Kenneth R. Laker & Willy M.C. Sansen, Design of Analog Integrated Circuits and Systems (New York: McGraw-Hill, Inc., 1994): pp. 456-462.
Referring now to FIG. 1, there is shown a circuit diagram of a conventional closed-loop microphone preamplifier 100, which is similar to that described in FIG. 6 of G. Nicollini et al., "A High-Performance Analog Front-End 14-Bit Codec for 2.7-V Digital Cellular Phones," IEEE Journal of Solid State Circuits, vol. 33, no. 8, August 1998, pp. 1158-1166. Microphone preamplifier 100 is a balanced differential op amp-based preamplifier, which comprises two symmetrical, balanced differential op amp circuits, each coupled to an identical feedback resistor R.sub.f and input resistor R.sub.i in a closed-loop configuration. Both input resistors R.sub.j are coupled at one terminal to common-mode voltage V.sub.cm, and at the other terminal to the negative input of their respective op amp and through feedback resistor R.sub.f to the output terminal of the respective op amp.
Common-mode voltage V.sub.CM is provided by a common-mode feedback (CMFB) circuit (not shown). CMFB circuits are described in further detail in the David A. Johns & Ken Martin text, at pp. 280-282. The CMFB circuitry is used to establish the common-mode (i.e., average) output voltage. Ideally, it keeps this common-mode voltage immovable, preferably close to halfway between the power-supply voltages (V.sub.DD) which power each op amp, even when large differential signals are present. Without it, the common-mode voltage is left to drift, since, although the op amps are placed in a feedback configuration, the common-mode loop gain is not typically large enough to control its value. Such is not the case with differential signals as the differential loop gain is typically quite large.
An input voltage signal V.sub.I is received in the form of differential input signals V.sub.IP, and V.sub.IN (where the letters P and N denote "positive" and "negative", where the "negative" signal V.sub.IN subtracted from the "positive" signal V.sub.IP represents the input signal V.sub.1). These differential input signals are applied to the positive input terminals of the two op amps, respectively. The differential input signals V.sub.IP and V.sub.IN may be considered to be generated from the components modeling the microphone transducer, namely an external voltage source V.sub.1, and its associated resistance R.sub.1 and capacitance C.sub.1. The preamplified output signal V.sub.O is the difference between differential outputs V.sub.OP and V.sub.ON.
In preamplifier 100, the gain A of each differential op amp circuit is R.sub.f /R.sub.i. The output voltage V.sub.O =V.sub.OP -V.sub.ON =AV.sub.I =A(V.sub.IP -V.sub.IN), where A=R.sub.f /R.sub.i. For a given gain A, the R.sub.f /R.sub.i ratio cannot be varied. However, different values of R.sub.f and R.sub.i may be selected to provide the same R.sub.f /R.sub.i ratio. In order to minimize noise, it is desirable to make the values of R.sub.f and R.sub.i as small as possible, because less noise is introduced for smaller resistances and for the higher feedback currents accompanying the use of smaller resistances. The values of R.sub.f and R.sub.i may be decreased so long as the desired ratio R.sub.f /R.sub.i is maintained to achieve gain A.
However, resistors R.sub.f and R.sub.i cannot be made arbitrarily small, because they pose loading constraints to the op amps. The smaller values for R.sub.f and R.sub.i selected, the larger current must be handled by the op amps. Thus, there is a limit to how small gain-setting resistors R.sub.f, R.sub.i can be, because smaller resistors introduce more load on the op amps. Accordingly, it may be impractical or impossible to reduce the noise introduced by the gain-setting resistors (primarily R.sub.i) low enough for low-noise applications such as microphone preamplifiers. Thus, when using closed-loop microphone preamplifiers, the noise introduced in the preamplifier may become dominant.